1. Field of the Invention
This invention relates to a system for driving an inductive load.
2. Description of the Related Art
Conventionally, a system that drives an inductive load such as an electromagnetic solenoid or electric motor is configured to turn on and off a switching element inserted in a location between an external power supply and the inductive load in an energization circuit of the inductive load in response to a pulse signal, so as to control energization from the external power supply to the inductive load and conduct duty (PWM) drive of the inductive load. Although, when energization to the inductive load is cut off, counter electromotive voltage is generated by the inductive load, counter electromotive current generated thereby is returned to a ground through a flywheel diode in order to prevent the energization circuit from being damaged by the counter electromotive voltage, as taught, for example, in Japanese Laid-Open Patent Application No. Hei 5(1993)-8413, especially in FIG. 1.
Since the known system is also equipped with a counter electromotive force absorbing circuit that absorbs counter electromotive voltage using a zener diode for attenuating the returned counter electromotive current and is configured to turn on and off a second switching element inserted in a position downstream of the inductive load, it can control return of counter electromotive current and absorption of counter electromotive voltage, thereby enabling to achieve a desired attenuation characteristic with respect to the energization amount of the inductive load after cutting off energization from the external power supply.
However, when counter electromotive voltage equal to or greater than a predetermined value (i.e., a breakdown voltage value of the zener diode) affects, the above-mentioned counter electromotive force absorbing circuit is conducted and absorbs counter electromotive voltage by an amount corresponding to the predetermined value. It causes a disadvantage to make the system itself overheat and the overheat leads to malfunction of the inductive load driving system.